Add scala adapters for doOnEach operator.
Remove unnecessary cast.
Merge remote-tracking branch 'upstream/master'
Merge branch 'master' into scala-do-operator
Remove unnecessary type parameter.

Apparently (to my surprise), the implicit conversions are smart enough
to resolve this.
Merge pull request #537 from landonf/scala-do-operator

Add scala adapters for doOnEach operator.